Added some comments about dependencies to list-ghsa

Signed-off-by: Mikaël Barbero <mikael.barbero@eclipse-foundation.org>

github/list-ghsa.sh

@@ -3,6 +3,11 @@
 # Before calling this script, you need to set the following environment variables:
 # $ export GH_TOKEN="ghp_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
 
+# requires:
+# * jq (https://stedolan.github.io/jq/)
+# * gh (https://cli.github.com/)
+# * jsonnet (https://jsonnet.org/)
+
 set -euo pipefail
 IFS=$'\n\t'